Ethylene is prepared commercially by heating ethane, propane, higher paraffins or naphtha, diluted with steam, at about 850.degree. C., 1550.degree. F., for very short contact times, without a catalyst. Highest ultimate yields come from ethane (81%), propane (43%) and n-butane (41.5%). All world-scale plants with billion-pound-per-year ethylene capacity are based on this thermal cracking/dehydrogenation technology. Although a host of rival schemes has been studied, none have reached commercial application.
Weaknesses in the established process are (1) high reaction temperature and low hydrocarbon partial pressure, (2) low product separation/purification temperatures (-150.degree. to -200.degree. F.) and high pressure (500 psig), (3) relatively low yields from C.sub.3 and higher feeds, (4) a complex mixture of products, and (5) relatively high capital and operating costs.
OIefins have been prepared from methanoI over ZSM-5 with low activity, SiO.sub.2 /Al.sub.2 O.sub.3 300/1, M. M. Wu and W. W. Kaeding, J. Cat. 88 478 (1984). In the majuor C.sub.2 -C.sub.4 olefins product, ethylene is usually the smallest component (10-15 wt %). When n-butane was used with these same catalysts, propylene and C.sub.5 + olefins were produced with only traces of ethylene. When propane is converted over catalysts with various oxides on silica or alumina such as chromium oxide, propylene is the major product.